This invention concerns copolymer beads suitable as a support matrix for ion-exchange resins and polymeric adsorbents. In particular, the invention concerns porous copolymer beads prepared by a seeded polymerization process.
Ion-exchange resins are used by industry to separate chemical species from solutions containing them. Such resins are prepared by substituting various functional groups onto a copolymer bead matrix. The functional groups are capable of associating with the impurities so as to remove them from solution. Ion-exchange resins may be cation-, anion- or chelate-exchange resins, depending on the choice of functional group substituted onto the copolymer bead matrix. The copolymer bead matrix may also be used in preparing polymeric adsorbents, such as those disclosed in U.S. Pat. No. 4,950,332.
The copolymer bead matrix is typically prepared by suspension polymerization of a finely divided organic phase comprising monovinylidene monomers like styrene, polyvinylidene monomers such as divinylbenzene, a freeradical initiator, and, optionally a phase-separating diluent. The copolymer beads produced may be microporous, i.e., gellular in character, or macroporous, the result depending upon whether the phase-separating diluent is employed. The term "macroporous" as commonly used in the art means that the copolymer has both macropores and mesopores. The terms "microporous", "gel" and "macroporous" are well-known in the art and generally describe the nature of the copolymer bead porosity. Microporous or gel copolymer beads have pore sizes on the order of less than about 20 Angstroms (.ANG.), while macroporous copolymer beads have both mesopores of from about 20 .ANG. to about 500 .ANG. and macropores of greater than about 500 .ANG.. Gel and macroporous copolymer beads, as well as their preparation, are further discussed in U.S. Pat. No. 4,256,840.
Recently, industry has focused on methods of preparing copolymer beads in multi-staged polymerizations, also known as seeded polymerization processes. Such copolymer beads are desirable due to good mechanical strength and osmotic shock resistance. Seeded polymerization processes can also prepare beads of relatively uniform size if the seed particles employed have similar size uniformity. U.S. Pat. Nos. 4,419,245 and 4,564,644 disclose processes wherein seed particles are suspended in a continuous medium and, thereafter, the seed particles are imbibed and polymerized with additional monomer to form copolymer beads. British Patent 1,151,480 discloses a process wherein copolymer beads are prepared from porous seed particles.
Other seeded polymerization processes are known. European Patent Application No. 0 062 088 (Oct. 13, 1982) discloses preparation of gel or macroporous copolymer beads by a seeded polymerization process. However, macroporous copolymer beads made from gel seeds according to this process do not have stable polymeric structures and generally are heterogeneous in porosity, i.e., some discrete portions of the beads are gellular, while other portions are macroporous. Such heterogeneous beads are undesirable due to poor physical strength. European Patent Application No. 0 168 622 (Jan. 22, 1986) concerns preparation of macroporous copolymer beads by a seeded polymerization process using macroporous seed particles. This process is limited by its inability to substantially increase the size of the copolymer bead product relative to the size of the seed particles.
In conducting a seeded polymerization, it is more advantageous to employ gel rather than macroporous seed particles. Gel seed particles, especially those with low levels of crosslinking monomer therein, i.e., less than about 5 weight percent based on weight of the seed particle, have a substantially better capability for imbibing additional monomers in a subsequent polymerization step. The greater ability to imbibe monomers allows for production of a copolymer bead product having a substantially greater volume, i.e., at least about 300 percent larger in size, relative to the initial size of the gel seed particle. On the other hand, when macroporous seed particles are employed it is difficult to obtain a significant increase in particle size and the pores of the seed particle are constricted by formation of new polymer.
As can be seen, it is desirable to develop a seeded polymerization process for producing porous copolymer beads which is economical and capable of preparing beads having a stable polymeric structure and good physical properties. The resulting porous copolymer beads could then be used to prepare ion-exchange resins and polymeric adsorbents.